In the semiconductor industry it has recently been discovered that some strain in switching transistors, such as FETs and the like, can actually increase the speed of the transistor. For example it has been found that compressive strain on the channel or active layer of a silicon transistor (i.e. a transistor formed on a silicon substrate), e. g. an MOS transistor, increases the number of available electrons and reduces the number of available holes, which increases the speed. Similarly, a tensile strain on the channel or active layer of a silicon transistor reduces the number of available electrons and increases the number of available holes, which reduces the speed.
In one attempt to increase the speed of transistors, an encapsulation or passivation layer is deposited over the transistor. Germanium is then implanted in the encapsulation layer to create strain in the transistor. The major problem is that the amount of strain is minimal and difficult to accurately adjust. Also, the germanium has a tendency to migrate into the transistor and change the characteristics and reduce the life.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide new and improved strained transistors.
Another object of the invention is to provide new and improved strained transistors with a stress engineered cap layer to produce an active layer with a desired amount of compressive stress, a desired amount of tensile stress, or no stress.
Another object of the invention is to provide new and improved strained transistors that can be formed very easily and with accurately engineered stress.
And another object of the invention is to provide a new and improved method of fabricating strained transistors.